The present invention is directed to a proximity switch assembly particularly adapted for use in signaling the location of a movable member at either of two end limits of movement. Although not so limited in application, the switch assembly of the present invention is especially adapted for use in power actuated clamps of the general type shown in my prior patent 4,396,183 in which a clamping arm is driven in movement between a closed workpiece clamping position and an open position by the piston rod of a fluid pressure actuated motor.
Such clamps are widely employed in automotive aassembly plants to clamp major sheet metal panels, such as roof panels, body side panels, and floor panels in a fixed position while the panels are being welded to each other. In the so called "framing station" disclosed in U.S. Pat. No. 4,162,387 a basic vehicle body assembly constituted by a floor panel, opposite side panels and a roof panel loosely assembled to each other is advanced into the framing station between a pair of clamp carrying "gates" which are then advanced to fixedly clamp the opposed side panels to locate the floor, side and roof panels in their final assembled position. While so clamped, robotic welding devices weld the panels to each other.
Systems such as that disclosed in U.S. Pat. No. 4,162,387 employ numerous power actuated clamps of the type referred to above, and in this completely automated system it is essential that all clamps be closed prior to the welding operation, and that this fact be verified by the control system before the robotic welders begin their welding cycle. Similarly, the control system must verify that all clamps are open before the retraction of the gates to accommodate the discharge of the welded body from the framing station.
While the detection of a clamp in its open or closed position could be accomplished by a conventional limit switch, the service life and reliability of exposed mechanically actuated limit switches in a robotic welding environment is unsatisfactory.
It has been proposed, see U.S. Pat. No. 4,316,145, to employ a commercially available magnetically biased reed switch, generally referred to as a proximity switch, mounted in the clamp actuating hydraulic cylinder to detect the arrival of the piston at either end of its stroke. The latter arrangement presents the advantage that the switch itself, due to its mounting in the cylinder, is shielded from dirt, weld splatter, etc., and the switch does not require any direct mechanical contact with a moving part in order to generate the desired signal. However, the arrangement disclosed in patent 4,316,145 requires the employment of two proximity switches, one to sense the arrival of the piston at each end of its stroke, the cylinder end cap must be modified to provide a mounting for the switch and normally a junction housing must be mounted on the exterior of the cylinder for each proximity switch. As noted above, in robotic welding systems, numerous clamp assemblies must be employed and mounted upon the gate at locations determined by the configuration of the body panel, rather than convenience, and substantial operating clearances must be provided for movement of several robotic welding heads which must pass through the gate to reach the seams which they are to weld.
The present invention is directed to a proximity switch assembly useful in the environment described above which is of extremely compact construction, requires a minimum of structural modification of a conventional power actuated clamp, and which requires only a single proximity switch whose contacts are located in one position when the clamp is clamped and remain in that one position until the clamp is completely open, at which time the contacts are shifted to their other position until the clamp is again at its clamped position.